CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Microtubule Reorganization and Quiescence: an Intertwined Relationship

Quiescence is operationally defined as a reversible proliferation arrest. This cellular state is central to both organism development and homeostasis, and its dysregulation causes many pathologies. The quiescent state encompasses very diverse cellular situations depending on the cell type and its environment. Further, quiescent cell properties evolve with time, a process that is thought to be the origin of aging in multicellular organisms. Microtubules are found in all eukaryotes and are essential for cell proliferation as they support chromosome segregation and intracellular trafficking. Upon proliferation cessation and quiescence establishment, the microtubule cytoskeleton was shown to undergo significant remodeling. The purpose of this review is to examine the literature in search of evidence to determine whether the observed microtubule reorganizations are merely a consequence of quiescence establishment or if they somehow participate in this cell fate decision.

Transcriptomic and ionomic analysis provides new insight into the beneficial effect of Al on tea roots' growth and nutrient uptake

Transcriptome profiling of roots indicated that genes involved in cell wall modification, cytoskeleton, H⁺ exchange and K⁺ influx played important roles in tea root growth under Al addition. Tea (Camellia sinensis) is considered as an Al accumulator species. It can accumulate a high concentration of Al in mature leaves without any symptom of toxicity, even improve roots' growth and nutrient uptake. However, the molecular mechanisms underlying this tolerance remain unclear. Here, we investigated the accumulation of elements and transcriptional profiles in tea roots treated with various Al doses. The results showed that the growth of tea plants was improved by a low dose of Al (0.2, 0.4, 0.6, 1 mM); however, this beneficial effect disappeared when higher concentrations of Al were supplied (2, 4, 10 mM). Ionomic analysis suggested that accumulation of P and K increased under a low Al supply (< 1 mM), while Ca and Mg contents were negatively correlated with external Al doses. The RNA seq obtained 523,391 unigenes, among which 20,448 were annotated in all databases. In total, 1876 unigenes were expressed significantly different in any Al treatment. A large number of DEGs involved in cell growth and division, such as those linked to cell wall-modifying enzymes, actin cytoskeleton, cyclin and H⁺-ATPase were identified, suggesting that these pathways were involved in root growth under different Al supply. Furthermore, expression of transporters significantly changed in roots supplied with Al. Among them, HAK5, which is involved in K uptake by plants, had a significant positive correlation with the K content.

Root cap-mediated evaluation of soil resistance towards graviresponding roots of maize (Zea mays L.) and the relevance of ethylene

BACKGROUND AND AIMS

Besides biological and chemical impacts, mechanical resistance represents an important obstacle that growing roots face. Graviresponding roots must assess the mechanical resistance of the substrate and take decisions on whether they change growth direction and grow around obstacles or tolerate growth conditions impaired to varying degrees. To test the significance of the root cap, we measured pressure and growth behaviour of single intact, as well as decapped, roots encountering diverse mechanical obstacles. We examined ethylene emission in intact roots as well as roots without a root cap, thereby lacking the capacity to deviate.

METHODS

Roots of fixed seedlings were grown vertically onto diverse mechanical obstacles. Developing pressure profiles of vertically growing roots encountering horizontal mechanical obstacles were measured employing electronic milligram scales, with and without root caps in given local environmental conditions. The evolution of root-borne ethylene was measured in intact roots and roots without the root cap.

KEY RESULTS

In contrast to decapped roots, intact roots develop a tentative, short-lasting pressure profile, the resolution of which is characterized by a definite change of growth direction. Similarly, pressure profiles and strengths of roots facing gradually differing surface resistances differ significantly between the two. This correlates in the short term with root cap-dependent ethylene emission which is lacking in roots without caps.

CONCLUSIONS

The way gravistimulated and graviresponding roots cope with exogenous stimuli depends on whether and how they adapt to these impacts. With respect to mechanical hindrances, roots without caps do not seem to be able to evaluate soil strengths in order to respond adequately. On encountering resistance, roots with intact caps emit ethylene, which is not observed in decapped roots. It therefore appears that it is the root cap which specifically orchestrates the resistance needed to overcome mechanical resistance by specifically inducing ethylene.

Single-point ACT2 gene mutation in the Arabidopsis root hair mutant der1-3 affects overall actin organization, root growth and plant development

BACKGROUND AND AIMS

The actin cytoskeleton forms a dynamic network in plant cells. A single-point mutation in the DER1 (deformed root hairs1) locus located in the sequence of ACTIN2, a gene for major actin in vegetative tissues of Arabidopsis thaliana, leads to impaired root hair development (Ringli C, Baumberger N, Diet A, Frey B, Keller B. 2002. ACTIN2 is essential for bulge site selection and tip growth during root hair development of Arabidopsis. Plant Physiology129: 1464-1472). Only root hair phenotypes have been described so far in der1 mutants, but here we demonstrate obvious aberrations in the organization of the actin cytoskeleton and overall plant development.

METHODS

Organization of the actin cytoskeleton in epidermal cells of cotyledons, hypocotyls and roots was studied qualitatively and quantitatively by live-cell imaging of transgenic lines carrying the GFP-FABD2 fusion protein and in fixed cells after phalloidin labelling. Patterns of root growth were characterized by FM4-64 vital staining, light-sheet microscopy imaging and microtubule immunolabelling. Plant phenotyping included analyses of germination, root growth and plant biomass.

KEY RESULTS

Speed of germination, plant fresh weight and total leaf area were significantly reduced in the der1-3 mutant in comparison with the C24 wild-type. Actin filaments in root, hypocotyl and cotyledon epidermal cells of the der1-3 mutant were shorter, thinner and arranged in more random orientations, while actin bundles were shorter and had altered orientations. The wavy pattern of root growth in der1-3 mutant was connected with higher frequencies of shifted cell division planes (CDPs) in root cells, which was consistent with the shifted positioning of microtubule-based preprophase bands and phragmoplasts. The organization of cortical microtubules in the root cells of the der1-3 mutant, however, was not altered.

CONCLUSIONS

Root growth rate of the der1-3 mutant is not reduced, but changes in the actin cytoskeleton organization can induce a wavy root growth pattern through deregulation of CDP orientation. The results suggest that the der1-3 mutation in the ACT2 gene does not influence solely root hair formation process, but also has more general effects on the actin cytoskeleton, plant growth and development.

Localization of calreticulin and calcium ions in mycorrhizal roots of Medicago truncatula in response to aluminum stress

Aluminum (Al) toxicity limits growth and symbiotic interactions of plants. Calcium plays essential roles in abiotic stresses and legume-Rhizobium symbiosis, but the sites and mechanism of Ca²⁺ mobilization during mycorrhizae have not been analyzed. In this study, the changes of cytoplasmic Ca²⁺ and calreticulin (CRT) in Medicago truncatula mycorrhizal (MR) and non-mycorrizal (NM) roots under short Al stress [50 μM AlCl₃ pH 4.3 for 3 h] were analyzed. Free Ca²⁺ ions were detected cytochemically by their reaction with potassium pyroantimonate and anti-CRT antibody was used to locate this protein in Medicago roots by immunocytochemical methods. In MR and NM roots, Al induced accumulation of CRT and free Ca²⁺. Similar calcium and CRT distribution in the MR were found at the surface of fungal structures (arbuscules and intercellular hyphae), cell wall and in plasmodesmata, and in plant and fungal intracellular compartments. Additionally, degenerated arbuscules were associated with intense Ca²⁺ and CRT accumulation. In NM roots, Ca²⁺ and CRT epitopes were observed in the stele, near wall of cortex and endodermis. The present study provides new insight into Ca²⁺ storage and mobilization in mycorrhizae symbiosis. The colocalization of CRT and Ca²⁺ suggests that CRT is essential for calcium mobilization for normal mycorrhiza development and response to Al stress.

Effect of short-term aluminum stress and mycorrhizal inoculation on nitric oxide metabolism in Medicago truncatula roots

Aluminum (Al) toxicity can induce oxidative and nitrosative stress, which limits growth and yield of crop plants. Nevertheless, plant tolerance to stress may be improved by symbiotic associations including arbuscular mycorrhiza (AM). Nitric oxide (NO) is a signaling molecule involved in physiological processes and plant responses to abiotic and biotic stresses. However, almost no information about the NO metabolism has been gathered about AM. In the present work, Medicago truncatula seedlings were inoculated with Rhizophagus irregularis, and 7-week-old plants were treated with 50μM AlCl₃ for 3h. Cytochemical and molecular techniques were used to measure the components of the NO metabolism, including NO content and localization, expression of genes encoding NO-synthesis (MtNR1, MtNR2 and MtNIR1) and NO-scavenging (MtGSNOR1, MtGSNOR2, MtHB1 and MtHB2) enzymes and the profile of protein tyrosine nitration (NO₂-Tyr) in Medicago roots. For the first time, NO and NO₂-Tyr accumulation was connected with fungal structures (arbuscules, vesicles and intercellular hyphae). Expression analysis of genes encoding NO-synthesis enzymes indicated that AM symbiosis results in lower production of NO in Al-treated roots in comparison to non-mycorrhizal roots. Elevated levels of transcription of genes encoding NO-scavenging enzymes indicated more active NO scavenging in AMF-inoculated Al-treated roots compared to non-inoculated roots. These results were confirmed by less NO accumulation and lower protein nitration in Al-stressed mycorrhizal roots in comparison to non-mycorrhizal roots. This study provides a new insight in NO metabolism in response to arbuscular mycorrhiza under normal and metal stress conditions. Our results suggest that mycorrhizal fungi decrease NO and tyrosine nitrated proteins content in Al-treated Medicago roots, probably via active NO scavenging system.

On Having No Head: Cognition throughout Biological Systems

The central nervous system (CNS) underlies memory, perception, decision-making, and behavior in numerous organisms. However, neural networks have no monopoly on the signaling functions that implement these remarkable algorithms. It is often forgotten that neurons optimized cellular signaling modes that existed long before the CNS appeared during evolution, and were used by somatic cellular networks to orchestrate physiology, embryonic development, and behavior. Many of the key dynamics that enable information processing can, in fact, be implemented by different biological hardware. This is widely exploited by organisms throughout the tree of life. Here, we review data on memory, learning, and other aspects of cognition in a range of models, including single celled organisms, plants, and tissues in animal bodies. We discuss current knowledge of the molecular mechanisms at work in these systems, and suggest several hypotheses for future investigation. The study of cognitive processes implemented in aneural contexts is a fascinating, highly interdisciplinary topic that has many implications for evolution, cell biology, regenerative medicine, computer science, and synthetic bioengineering.

Root apex transition zone as oscillatory zone

Root apex of higher plants shows very high sensitivity to environmental stimuli. The root cap acts as the most prominent plant sensory organ; sensing diverse physical parameters such as gravity, light, humidity, oxygen, and critical inorganic nutrients. However, the motoric responses to these stimuli are accomplished in the elongation region. This spatial discrepancy was solved when we have discovered and characterized the transition zone which is interpolated between the apical meristem and the subapical elongation zone. Cells of this zone are very active in the cytoskeletal rearrangements, endocytosis and endocytic vesicle recycling, as well as in electric activities. Here we discuss the oscillatory nature of the transition zone which, together with several other features of this zone, suggest that it acts as some kind of command center. In accordance with the early proposal of Charles and Francis Darwin, cells of this root zone receive sensory information from the root cap and instruct the motoric responses of cells in the elongation zone.

NIMA-related kinases regulate directional cell growth and organ development through microtubule function in Arabidopsis thaliana

NIMA-related kinase 6 (NEK6) regulates cellular expansion and morphogenesis through microtubule organizaiton in Arabidopsis thaliana. Loss-of-function mutations in NEK6 (nek6/ibo1) cause ectopic outgrowth and microtubule disorganization in epidermal cells. We recently found that NEK6 forms homodimers and heterodimers with NEK4 and NEK5 to destabilize cortical microtubules possibly by direct binding to microtubules and the β-tubulin phosphorylation. Here, we identified a new allele of NEK6 and further analyzed the morphological phenotypes of nek6/ibo1 mutants, along with alleles of nek4 and nek5 mutants. Phenotypic analysis demonstrated that NEK6 is required for the directional growth of roots and hypocotyls, petiole elongation, cell file formation, and trichome morphogenesis. In addition, nek4, nek5, and nek6/ibo1 mutants were hypersensitive to microtubule inhibitors such as propyzamide and taxol. These results suggest that plant NEKs function in directional cell growth and organ development through the regulation of microtubule organization.

The 'root-brain' hypothesis of Charles and Francis Darwin: Revival after more than 125 years

This year celebrates the 200(th) aniversary of the birth of Charles Darwin, best known for his theory of evolution summarized in On the Origin of Species. Less well known is that, in the second half of his life, Darwin's major scientific focus turned towards plants. He wrote several books on plants, the next-to-last of which, The Power of Movement of Plants, published together with his son Francis, opened plants to a new view. Here we amplify the final sentence of this book in which the Darwins proposed that: "It is hardly an exaggeration to say that the tip of the radicle thus endowed [with sensitivity] and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements." This sentence conveys two important messages: first, that the root apex may be considered to be a 'brain-like' organ endowed with a sensitivity which controls its navigation through soil; second, that the root apex represents the anterior end of the plant body. In this article, we discuss both these statements.

Cylindrospermopsin induces alterations of root histology and microtubule organization in common reed (Phragmites australis) plantlets cultured in vitro

We aimed to study the histological and cytological alterations induced by cylindrospermopsin (CYN), a protein synthesis inhibitory cyanotoxin in roots of common reed (Phragmites australis). Reed is an ecologically important emergent aquatic macrophyte, a model for studying cyanotoxin effects. We analyzed the histology and cytology of reed roots originated from tissue cultures and treated with 0.5-40 microg ml(-1) (1.2-96.4 microM) CYN. The cyanotoxin decreased root elongation at significantly lower concentrations than the elongation of shoots. As general stress responses of plants to phytotoxins, CYN increased root number and induced the formation of a callus-like tissue and necrosis in root cortex. Callus-like root cortex consisted of radially swollen cells that correlated with the reorientation of microtubules (MTs) and the decrease of MT density in the elongation zone. Concomitantly, the cyanotoxin did not decrease, rather it increased the amount of beta-tubulin in reed plantlets. CYN caused the formation of double preprophase bands; the disruption of mitotic spindles led to incomplete sister chromatid separation and disrupted phragmoplasts in root tip meristems. This work shows that CYN alters reed growth and anatomy through the alteration of MT organization.

Pb/Cu effects on the organization of microtubule cytoskeleton in interphase and mitotic cells of Allium sativum L

The effects of lead and copper on the arrangement of microtubule (MT) cytoskeleton in root tip cells of Allium sativum L. were investigated. Batch cultures of garlic were carried out under defined conditions in the presence 10(-4) M Pb/Cu of various duration treatments. With tubulin immunolabelling and transmission electron microscopy (TEM), we found four different types of MT structures depending on the cell cycle stage: the interphase array, preprophase band, mitotic spindle and phragmoplast were typical for the control cells. Pb/Cu affected the mechanisms controlling the organization of MT cytoskeleton, and induces the following aberrations in interphase and mitotic cells. (1) Pb/Cu induced the formation of atypical MT arrays in the cortical cytoplasm of the interphase cells, consisting of skewed, wavy MT bundles, MT fragments and ring-like tubulin aggregations. (2) Pb/Cu disordered the chromosome movements carried out by the mitotic spindle. The outcome was chromosome aberrations, for example, chromosome bridges and chromosome stickiness, as well as inhibition of cells from entering mitosis. (3) Depending on the time of exposure, MTs disintegrated into shorter fragments or they completely disappeared, indicating MT depolymerization. (4) Different metals had different effects on MT organization. MTs were more sensitive to the pressure of Cu ions than Pb. Moreover, TEM observations showed that the MTs were relatively short and in some places wavy when exposed to 10(-4) M Pb/Cu solutions for 1-2 h. In many sections MTs were no longer visible with increasing duration of treatment (>4 h). Based on these results, we suggested that MT cytoskeleton is primarily responsible for Pb/Cu-associated toxicity and tolerance in plants.

Environmental effects on spatial and temporal patterns of leaf and root growth

Leaves and roots live in dramatically different habitats, but are parts of the same organism. Automated image processing of time-lapse records of these organs has led to understanding of spatial and temporal patterns of growth on time scales from minutes to weeks. Growth zones in roots and leaves show distinct patterns during a diel cycle (24 h period). In dicot leaves under nonstressful conditions these patterns are characterized by endogenous rhythms, sometimes superimposed upon morphogenesis driven by environmental variation. In roots and monocot leaves the growth patterns depend more strongly on environmental fluctuations. Because the impact of spatial variations and temporal fluctuations of above- and belowground environmental parameters must be processed by the plant body as an entire system whose individual modules interact on different levels, growth reactions of individual modules are often highly nonlinear. A mechanistic understanding of plant resource use efficiency and performance in a dynamically fluctuating environment therefore requires an accurate analysis of leaf and root growth patterns in conjunction with knowledge of major intraplant communication systems and metabolic pathways.

Cell-type-specific disruption and recovery of the cytoskeleton in Arabidopsis thaliana epidermal root cells upon heat shock stress

The cytoskeleton in plant cells plays an important role in controlling cell shape and mediating intracellular signalling. However, almost nothing is known about the reactions of cytoskeletal elements to heat stress, which represents one of the major environmental challenges for plants. Here we show that living epidermal root cells of Arabidopsis thaliana could cope with short-term heat shock stress showing disruption and subsequent recovery of microtubules and actin microfilaments in a time-dependent manner. Time-lapse imaging revealed a very dynamic behavior of both cytoskeletal elements including transient depolymerization and disassembly upon heat shock (40-41 degrees C) followed by full recovery at room temperature (20 degrees C) within 1-3 h. Reaction of microtubules, but not actin filaments, to heat shock was dependent on cell type and developmental stage. On the other hand, recovery of actin filaments, but not microtubules, from heat shock stress was dependent on the same parameters. The relevance of this adaptive cytoskeletal behavior to intracellular signalling is discussed.

Neutral red as a probe for confocal laser scanning microscopy studies of plant roots

BACKGROUND AND AIMS

Neutral red (NR), a lipophilic phenazine dye, has been widely used in various biological systems as a vital stain for bright-field microscopy. In its unprotonated form it penetrates the plasma membrane and tonoplast of viable plant cells, then due to protonation it becomes trapped in acidic compartments. The possible applications of NR for confocal laser scanning microscopy (CLSM) studies were examined in various aspects of plant root biology.

METHODS

NR was used as a fluorochrome for living roots of Phaseolus vulgaris, Allium cepa, A. porrum and Arabidopsis thaliana (wild-type and transgenic GFP-carrying lines). The tissues were visualized using CLSM. The effect of NR on the integrity of the cytoskeleton and the growth rate of arabidopsis primary roots was analysed to judge potential toxic effects of the dye.

KEY RESULTS

The main advantages of the use of NR are related to the fact that NR rapidly penetrates root tissues, has affinity to suberin and lignin, and accumulates in the vacuoles. It is shown that NR is a suitable probe for visualization of proto- and metaxylem elements, Casparian bands in the endodermis, and vacuoles in cells of living roots. The actin cytoskeleton and the microtubule system of the cells, as well as the dynamics of root growth, remain unchanged after short-term application of NR, indicating a relatively low toxicity of this chemical. It was also found that NR is a useful probe for the observation of the internal structures of root nodules and of fungal hyphae in vesicular-arbuscular mycorrhizas.

CONCLUSIONS

Ease, low cost and absence of tissue processing make NR a useful probe for structural, developmental and vacuole-biogenetic studies of plant roots with CLSM.

Differential expression profiles of growth-related genes in the elongation zone of maize primary roots

Growth in the apical elongation zone of plant roots is central to the development of functional root systems. Rates of root segmental elongation change from accelerating to decelerating as cell development proceeds from newly formed to fully elongated status. One of the primary variables regulating these changes in elongation rates is the extensibility of the elongating cell walls. To help decipher the complex molecular mechanisms involved in spatially variable root growth, we performed a gene identification study along primary root tips of maize (Zea mays) seedlings using suppression subtractive hybridization (SSH) and candidate gene approaches. Using SSH we isolated 150 non-redundant cDNA clones representing root growth-related genes (RGGs) that were preferentially expressed in the elongation zone. Differential expression patterns were revealed by Northern blot analysis for 41 of the identified genes and several candidate genes. Many of the genes have not been previously reported to be involved in root growth processes in maize. Genes were classified into groups based on the predicted function of the encoded proteins: cell wall metabolism, cytoskeleton, general metabolism, signaling and unknown. In-situ hybridization performed for two selected genes, confirmed the spatial distribution of expression shown by Northern blots and revealed subtle differences in tissue localization. Interestingly, spatial profiles of expression for some cell wall related genes appeared to correlate with the profile of accelerating root elongation and changed appropriately under growth-inhibitory water deficit.

A polarized cell: the root statocyte

In the gravity-perceiving cells (statocytes), located in the centre of the root cap, polarity is expressed in the arrangement of the organelles since, in most genera, the nucleus and the endoplasmic reticulum are maintained at the opposite ends of each cell by actin. Polarity is also evident in the distribution of plasmodesmata, which are more numerous in the transverse walls than in the longitudinal walls. The centre of each statocyte is depleted of microtubules (they are only located at the periphery) but is occupied by numerous amyloplasts (statoliths), denser than the cytoplasm. The amyloplasts do not contribute to the inherent structural polarity since their position is dependent upon the gravity vector. This article focuses on new microscopic analyses and on data obtained from experiments performed in microgravity, which have contributed to our better understanding of the architecture of the actin web implicated in the perception of gravity. Depending upon the plant, the actin network seems to be formed of single filaments arranged in various ways, or, of thin bundles of actin filaments. The amyloplasts are enmeshed in this web of actin and their envelopes are associated with it, but they can have autonomous movement via myosin in the absence of gravity. From calculations of the value of the force necessary to move one amyloplast in the lentil root, and from videomicroscopy performed with living statocytes of maize roots, it is hypothesized that actin microfilaments could be orientated in an overall diagonal direction in the statocyte. These observations could help in understanding how slight amyloplast movements may trigger and transmit the gravitropic signal.

Elevated levels of N-lauroylethanolamine, an endogenous constituent of desiccated seeds, disrupt normal root development in Arabidopsis thaliana seedlings

N-Acylethanolamines (NAEs) are prevalent in desiccated seeds of various plant species, and their levels decline substantially during seed imbibition and germination. Here, seeds of Arabidopsis thaliana (L.) Heynh. were germinated in, and seedlings maintained on, micromolar concentrations of N-lauroylethanolamine (NAE 12:0). NAE 12:0 inhibited root elongation, increased radial swelling of root tips, and reduced root hair numbers in a highly selective and concentration-dependent manner. These effects were reversible when seedlings were transferred to NAE-free medium. Older seedlings (14 days old) acclimated to exogenous NAE by increased formation of lateral roots, and generally, these lateral roots did not exhibit the severe symptoms observed in primary roots. Cells of NAE-treated primary roots were swollen and irregular in shape, and in many cases showed evidence, at the light- and electron-microscope levels, of improper cell wall formation. Microtubule arrangement was disrupted in severely distorted cells close to the root tip, and endoplasmic reticulum (ER)-localized green fluorescent protein (mGFP5-ER) was more abundant, aggregated and distributed differently in NAE-treated root cells, suggesting disruption of proper cell division, endomembrane organization and vesicle trafficking. These results suggest that NAE 12:0 likely influences normal cell expansion in roots by interfering with intracellular membrane trafficking to and/or from the cell surface. The rapid metabolism of NAEs during seed imbibition/germination may be a mechanism to remove this endogenous class of lipid mediators to allow for synchronized membrane reorganization associated with cell expansion.

Auxin deprivation induces a developmental switch in maize somatic embryogenesis involving redistribution of microtubules and actin filaments from endoplasmic to cortical cytoskeletal arrays

A developmental switch from non-polar pre-embryogenic units to polarized transition units in maize embryogenic callus is caused by auxin deprivation from the culture medium. This switch is accompanied by cytoskeletal rearrangements in embryogenic cells. An immunofluorescence study revealed prominent endoplasmic microtubules and actin filament meshworks radiating from the nuclear surfaces in pre-embryogenic cells growing on medium supplemented with auxin. On the other hand, parallel-organized cortical microtubules and cortical actin filament networks are inherently associated with polarized embryogenic cells of transition units growing on medium without auxin. These results indicate that fine-tuning of the dynamic equilibrium between endoplasmic and cortical cytoskeletal arrays is important for progress in somatic embryogenesis.

Tissue specific localization of root infection by fungal pathogens: role of root border cells

When roots of pea seedlings were inoculated uniformly with spores of Nectria haematocca or other pea pathogenic fungi, more than 90% developed lesions in the region of elongation within 3 days. More mature regions of most roots as well as the tip showed no visible signs of infection. Yet, microscopic observation revealed that 'mantles,' comprised of fungal hyphae intermeshed with populations of border cells, covered the tips of most roots. After physical detachment of the mantle, the underlying tip of most roots was found to be free of infection. Mantle-covered root tips did not respond to invasion of their border cells by activation of known defense genes unless there was invasion of the tip itself, as revealed by the presence of a lesion. Concomitant with the activation of defense genes was the induction of a cell-wall degrading enzyme whose expression is a marker for renewed production of border cells. Mantle formation did not occur in response to nonpathogens. The data are consistent with the hypothesis that border cells serve as a host-specific 'decoy' that protects root meristems by inhibiting fungal infection of the root tip.

Actin monoubiquitylation is induced in plants in response to pathogens and symbionts

Most dramatic examples of actin reorganization have been described during host-microbe interactions. Plasticity of actin is, in part, due to posttranslational modifications such as phosphorylation or ubiquitylation. Here, we show for the first time that actins found in root nodules of Phaseolus vulgaris are modified transiently during nodule development by monoubiquitylation. This finding was extended to root nodules of other legumes and to other plants infected with mycorrhiza or plant pathogens such as members of the genera Pseudomonas and Phytophthora. However, neither viral infections nor diverse stressful conditions (heat shock, wounding, or osmotic stress) induced this response. Additionally, this phenomenon was mimicked by the addition of a yeast elicitor or H2O2 to Phaseolus vulgaris suspension culture cells. This modification seems to provide increased stability of the microfilaments to proteolytic degradation and seems to be found in fractions in which the actin cytoskeleton is associated with membranes. All together, these data suggest that actin monoubiquitylation may be considered an effector mechanism of a general plant response against microbes.

Lilliputian mutant of maize lacks cell elongation and shows defects in organization of actin cytoskeleton

The maize mutant lilliputian is characterized by miniature seedling stature, reduced cell elongation, and aberrant root anatomy. Here, we document that root cells of this mutant show several defects in the organization of actin filaments (AFs). Specifically, cells within the meristem lack dense perinuclear AF baskets and fail to redistribute AFs during mitosis. In contrast, mitotic cells of wild-type roots accumulate AFs at plasma membrane-associated domains that face the mitotic spindle poles. Both mitotic and early postmitotic mutant cells fail to assemble transverse arrays of cortical AFs, which are characteristic for wild-type root cells. In addition, early postmitotic cells show aberrant distribution of endoplasmic AF bundles that are normally organized through anchorage sites at cross-walls and nuclear surfaces. In wild-type root apices, these latter AF bundles are organized in the form of symmetrically arranged conical arrays and appear to be essential for the onset of rapid cell elongation. Exposure of wild-type and cv. Alarik maize root apices to the F-actin drugs cytochalasin D and latrunculin B mimics the phenotype of lilliputian root apices. In contrast to AFs, microtubules are more or less normally organized in root cells of lilliputian mutant. Collectively, these data suggest that the LILLIPUTIAN protein, the nature of which is still unknown, impinges on plant development via its action on the actin cytoskeleton.

Arabidopsis thaliana Rop GTPases are localized to tips of root hairs and control polar growth

Plants contain a novel unique subfamily of Rho GTPases, vital components of cellular signalling networks. Here we report a general role for some members of this family in polarized plant growth processes. We show that Arabidopsis AtRop4 and AtRop6 encode functional GTPases with similar intrinsic GTP hydrolysis rates. We localized AtRop proteins in root meristem cells to the cross-wall and cell plate membranes. Polar localization of AtRops in trichoblasts specifies the growth sites for emerging root hairs. These sites were visible before budding and elongation of the Arabidopsis root hair when AtRops accumulated at their tips. Expression of constitutively active AtRop4 and AtRop6 mutant proteins in root hairs of transgenic Arabidopsis plants abolished polarized growth and delocalized the tip-focused Ca2+ gradient. Polar localization of AtRops was inhibited by brefeldin A, but not by other drugs such as latrunculin B, cytochalasin D or caffeine. Our results demonstrate a general function of AtRop GTPases in tip growth and in polar diffuse growth.

Aluminum-induced 1-->3-beta-D-glucan inhibits cell-to-cell trafficking of molecules through plasmodesmata. A new mechanism of aluminum toxicity in plants

Symplastic intercellular transport in plants is achieved by plasmodesmata (PD). These cytoplasmic channels are well known to interconnect plant cells to facilitate intercellular movement of water, nutrients, and signaling molecules including hormones. However, it is not known whether Al may affect this cell-to-cell transport process, which is a critical feature for roots as organs of nutrient/water uptake. We have microinjected the dye lucifer yellow carbohydrazide into peripheral root cells of an Al-sensitive wheat (Triticum aestivum cv Scout 66) either before or after Al treatment and followed the cell-to-cell dye-coupling through PD. Here we show that the Al-induced root growth inhibition is closely associated with the Al-induced blockage of cell-to-cell dye coupling. Immunofluorescence combined with immuno-electron microscopic techniques using monoclonal antibodies against 1-->3-beta-D-glucan (callose) revealed circumstantial evidence that Al-induced callose deposition at PD may responsible for this blockage of symplastic transport. Use of 2-deoxy-D-glucose, a callose synthesis inhibitor, allowed us to demonstrate that a reduction in callose particles correlated well with the improved dye-coupling and reduced root growth inhibition. While assessing the tissue specificity of this Al effect, comparable responses were obtained from the dye-coupling pattern in tobacco (Nicotiana tabacum) mesophyll cells. Analyses of the Al-induced expression of PD-associated proteins, such as calreticulin and unconventional myosin VIII, showed enhanced fluorescence and co-localizations with callose deposits. These results suggest that Al-signal mediated localized alterations to calcium homeostasis may drive callose formation and PD closure. Our data demonstrate that extracellular Al-induced callose deposition at PD could effectively block symplastic transport and communication in higher plants.